Traditionally, with the utility grid (hereinafter “grid”), all of the generation sources are large rotating machines, and the grid controls the balance between the load and generation by controlling the frequency of the grid. The rotating machines store a portion of the energy in the rotating masses thereof. This stored energy either absorbs excess energy from the grid by small increases in the speed of rotation or supplies energy to the grid by small decreases in the speed of rotation in conjunction with a generator. This results in small changes in the grid's frequency. Thus, the electrical energy produced by the grid's generators matches that of the electrical energy required by the system load.
As the world population size increases, generation of energy for the grid is moving to more and more renewables sources, such as solar and wind generators.
There have been discussions for the ability of the Western Interconnection to withstand a disturbance caused by the loss of a large generation source with a large amount of wind or solar photovoltaic technology on the Western Interconnection. In some simulations, frequency disturbance on the grid would come close to the load shed initiation frequency of 59.5 Hz. See, for example, FIG. 1, which shows the frequency response of the Western Interconnection with 44% penetration of solar and wind generation. This may occur when there is a temporary loss of power on the grid, e.g., at a large coal fired or nuclear power plant.
With the minimum frequency of 59.65 HZ, the system would be marginally stable. There has been further discussion that wind and solar could contribute to the system response to this type of disturbance by operating below maximum power conditions, which would allow the wind and solar power plants to increase their output in response to a frequency disturbance. If this were implemented, the time required to restore system frequency to 60 Hz would be reduced.
However, since wind and solar power generators are inertia-less generation sources, wind and solar power generators do not contribute to supplying energy to the grid by slowing down the rotating mass of a hydro or thermal turbine generator. For this reason, it has been suggested that grid support between the time of disturbance initiation and governor response by the remaining generators was to run displaced coal fired generation as synchronous condensers to make up for the lack of the ability of solar and wind generators to respond to the frequency disturbance in a similar manner to that of conventional hydro and thermal generation.
To prevent or mitigate these issues, an alternative approach may be beneficial.